Process of manufacturing fibrous sheet covered plywood



D. H. GRANsAA-RD PROCESS OF MANUFACTURING FIBROUS SHEET COVERED PLYWOODFiled June 5, 1951 oct. 2o, 1953 3 snets-sheet 1 Patented Oct. 20, 1953PROCESS F MANUFACTURING FIBROUS SHEET COVERED PLYWOOD Donald H.Grangaard, Appleton, Wis., assignor to Paper Patents Company, Neenah,Wis., a corporation of Wisconsin Application June 5, 1951, Serial No.229,951

The present invention relates generally to the manufacture of plywoodand similar sheet materials, and has for its principal object theimprovement of the surface characteristics of these materials,especially plywood made from Douglas fir.

, As will hereinafter appear, this object is accomplished by theprovision of a masking or cover sheet of novel type, which is adapted tobe attached to the sheet material with which it is used so as toconstitute the exposed surface thereof.

Very large amounts of plywood are manufactured and used each year in theUnited States for buildings and for other purposes, and a large portionof this plywood is made from Douglas fir. This wood' has the advantagesof being strong and relatively inexpensive, while at the same time beingcomparatively easy to fabricate. Douglas fir plywood has, however,certain very important disadvantages. Particularly, it is difiicult toobtain logs which are completely free from knots or other blemishes, andas a result, the available surface veneers frequently contain relativelylarge knot holes, pitch pockets or other defects. Moreover, even when itis possible to Obtain a substantially perfect surface ply, Douglas firis subject to checking and to very severe grain-raising, both of whichrender the obtaining of a smooth, grain-free finish on the plywoodsurface almost impossible.

The phenomenon of grain-raising is caused by the variation in density inthe wood due to differing rates of growth in the spring and summergrowth periods. The differing densities in the wood result in differingrates of expansion or contraction with change in moisture content, andthe grain becomes clearly visible on the face of the sheet or panel.This is especially objectionable in cabinets, doors, etc., which are tobe subsequently painted, or finished, with a glossy paint or enamel.

Checking is evidenced by relatively short, hair line cracks, the cracksdeveloping most rapidly when the face veneer is dried out, a conditionwhich commonly occurs in the interior of houses, etc.,when the relativehumid-ity is low. In aggravated cases, the wood may actually split. Itis not possible to completely prevent or overcome checking or splits bypainting, enameling, or

pear as cracks in the nish.

The defects resulting from knots, pitch pockets, and the like arecorrected by the use of plug patches, the most commonly used types being4 Claims. (Cl. 154-132) known as cookie patches and boat patches. In theuse of plug patches, a hole is cut or punched from the wood sheet orveneer of sufiicient size to include the entire defective area,l

and an insert of similar shape, which has been cut from a perfect sheet,is inserted in the cutout. These inserts are, however, clearly visiblein the completed plywood sheet, and since at least the outline will showthrough paint or other finishes, they have the effect of lowering thequality of the sheet. To this extent, the defects resulting from the useof plug patches are most serious, from an economic viewpoint, Vbecause aplywood sheet which contains any visible defects on either surface mustbe sold as a lower grade sheet than one which is completely free fromdefects.

In the practice of the present invention, a sheet of resin containingpaper is applied to 1 the exposed surface of the plywood. Thisresinbearing sheet has certain very definite requirements andcharacteristics, as will hereinafter appear, and when applied to theplywood surface, lit

effectively masks all of the ordinary surface de-v fects, includingraised grain, ordinary checking and small splits, and both cookie andboat type plug patches. The principal object of the in.- Vention is thusaccomplished and a smooth yfinish, free from grain or otherimperfections is obtained on the plywood or other sheet material withwhich the masking sheet is used.

The present application is a continuation-inpart of my prior abandonedapplications Serial No. 779,889, and Serial No. 116,810, which werefiled on October 15, 1947, and September 20, 1949, respectively, andwhich were assigned to the assignee of the present invention. In thoseapplications I have disclosed certain particularly satisfactory maskingsheetsA which are likewise capable of accomplishing the above statedprincipal object of the invention. More particularly, the earlier ledapplication, Serial No. 779,889, is principally directed to maskingsheets containing resins of the phenol-aldehyde type, while the secondapplication, Serial No. 116,810, is concerned primarily with maskingsheets containing 'other nishing methods, and these defects apresins ofthe urea-aldehyde type.

This application combines the disclosures of these two priorapplications, and includes, 'in addition, certain extensions of, andadditions to, the disclosures madeV these said applications,particularly with regard to the addition of the resin to the sheet.Hence, a further object of the present invention is to provide improvedmethods o f manufacturing resin-containing ply- `wood masking sheets, inaccordance with the 'general principle disclosed in my said applicationsSerial No. 779,889 and Serial No. 116,810. A still further, and evenmore specific object of the invention is to produce a masking sheet ofthis type which contains one or morenf the beater addition resins',alone o r in` combination With one or more other resins and which, atthe same time, has very high wet abrasion and scuff resisting surfaces.

In Figure 1 of the drawings; there s'hematic illustration of a method ofmanufacturing the improved cover or masking sheet of the invention;

Figure 2 shows, in schematic illustration, another method ofmanufacturing the improved masking sheet of the invention;

Figure 3 is an enlarged, fragxnentaryperspe tive View, showing thecompleted masking sheet having. e glee-lirrrler-erhneeive @replied.the-rete; ....Eisurefr free eieererrmetwiew .Off .e ply-W09@ mess andillustrates .the meenerirr rb ierrmesk? irrasheets..irr-eeeerderreewithiihe.irrirfrriierl'rlei' besepplied .te the,..s1;1eeee Oiihe. .Plrweed panels.; .Figure 5. is .a .seetreeel newillustrating.. e. Per: of e. elifweee .Darrel-with.- eerer .Qrmeslrirlesheets. `in accorda nce .vvith the invention, app1ieslf..te. beth.serfeees .heref;. Y

.1.. rieure elan vrevmf e. eie.rr1...mber rise@ in.. determining..eertein ...ebereeteristes 0f the masking sheets eff the irri/entier;

f Figure? ,is en.. enlarged. iraginentary.. seerienal new....illustratirrehemanner .in which the die memberef Fieur.e8..i.saerephehQwi.. e....verieiir1. in the per cent of extractable resin Withur ing .time ron-three .representative .types ef.. .meskineehets made inaccordanoe v vith thepresent invention from,e. .'relativelyrs.1.evf....am,edir1rrry.er1d e feet curing,.resir.1..,..-...."1,`he..e'reeh also., .illustrates .e .resiprocedureyvhich will hereinafter be described ir;sreater...rietei 1....,ln...the.merruieeture ...er .the 'meshing street .Qf thepresentinvention... .resin is uniformly distrib.- uted .threuehoutthesheet andthsresinis. then advanced to. a. high. .degreeef eure. ,the ed weer?desreenof .eure being.. V011e ior.. the imperia?? .iee- .tures ,orlthe..inventier1 ...The resrnrnerhe ubi.-

iormlynistributed.threusheut e besesheetlefier that sheethasbeenformedaonirr .the alternative., theresrr may .be introduced.. .inteelle .Dreduet-by mixng..a.resin..or ...resins .with .the 9.1.1.1r,...,The masking sheet...ey.also.:bemenufeetrlrerlby dis.-ibutng...the.resin. or-.resns irl the sheet by e eombnatonof..these,methods....m.. M .when in@ -resin is. to be. .irrtrodueedinieerre,- iormed. sheet, ...n..bese sheet. 0fy .rather moderately..absorbent .paper,.-,mey be -peseed through.resiny apply-ins .Ineens.seas te eieet .errirorm .distribution of .resin throughout thaesheet.-The. .resinfleeeurrlg.` sheet. .may..,the.n. be.. mei/eelthreuehdryirreend..eurir1e..erer1s tei. eerrlplisb eeritrelleci,@urina-.0r the.. resin- Felewirre.. the curing.eneratieaiheresin.,bearing.,eheetlrleS/.Pe

.conducted threuehaeeolmazene, after, .whieh .er1 adhesive coatingmay.be applied to Qnesurfae thereetier. the purpese-frrerldrle glue; line-As a final s tep, .the glue ,..if used, 'should be dried. .teenereteekr.. eeneitierlf If` ainpr.part.lof,..the.resirris.t9 be..impregnated inithe. masking .sheet as errineident,tethemermf .facture.of .the .leasey sheet,..e beatervedeliten. .typeresin..,whiehhesrelatively v.reuiirl eurngnehereeteristies,isfintrosiueed into .the peper furnish preferably at the beater, theresin being in either liquid or solid form. When this is done, thebeater thoroughly disperses and uniformly distributes the resinthroughout the pulp ber. The furnish is then pumped to a conventionaltype paper-making machine, such as a Fourdrinier machne, andformedintoga sheet. A glue vline iriaybefpplidltothe sheet atthe end ofthe paper making operation and, if the g1ue line is used, it is driedsuiiciently to produce a non-tacky condition as a nal step in theformatioilbfwthewfrs'liig sheet of the invention.

'Rgardless` of the particular method employed i0 ierreet-rheintnqgnuonof resin into the sheet, itis portanti-hat the resin shall be uniformlydistributed throughout the sheet, and that it shall be curedlto ith ee,xtent to be hereinafter defined. When this isudne, the sheetsresulting from any ofhtheseunethods Will have excellent maskingcharacteristics. In this;v connection, it should be noted that. .thepreferredinethods of manufacturing sinfbeariigfhiaskng sheetsin `acrdancez'wthuthe invention d'o not use press efoi the purpselef df 'ingtheresin eure. aridi @i .bhe .resmfeureisi .eereeeedion hier. 'degree 'eer to lany use of thev masking sheet. The latter chai?acteristiclothe.-,invention is of signiancej'r thereafsonthatitisco'ntrary'to the commercial practicel of preparing ,resinbearing,laminates and, indeed, has been carefully avoidedheretofore. Thedravving's Villustrate tivo general typ s of apparatus formanufacturingthe masking vsheet negentien. reuse. eiserne., ressenswherebythe resrl...`lrrillfe l'ation may be...in'tro` duced into'apreformed sheetand ,Figure '2,'.illu`s iretirisapparet's whereri .resinmay AJee infiel .duced intola tjf .paper .during @H manu'- factre ofLthat sheet. AIfrth'e. particular. apparatus illustrated v.in.,figure-.l, a base 'sheet is;` con- .tainedpn .1 supmyron, illustrated 'at 1 anduns `sheet lis guided tl1roug-h;` a pan 8 .which contains' a solutionfofimpregnatinaresin, by rreans. of fguidenrollsjandill). Ifo assurethatthe proper amount fofAresfn'nvvill be contained in the sheet,anditoeffect even. resin i; jip 'gnation and. diseieeeee. .e.perr'geffs'eleeze .51.1 end fillers arranged toengagejthebas'e 'she'tasIit emerges frrr-rnthe. batl i.o f resin.. .Je 'Squee'ie'rolls I Ierre... l?. ere desireels'z eldl eerily sbrierea'iri order that theamounnonresin .retention in .une vererben. .eerr...lee. eeerrtelv.'erirf'Hed. .After leavingthe s'iueeael rollsal I2, theshfeet is cnveyed through. an oven I3. by4 means of a v.Belte9r1veyer;.l.5....Theme- 'lineledes threaten@ rate...heating2 sections:|1 If'and` |9, which. are adapted.. tube maintained 'fat differentinter'de'-, renderer. een... ree@ .temeereferes in. order 'ther the..drying ...elle euries 'er the resin. rrlpreeeied erreeee'err Jee,eee'ereelr eenlrelled. ...Trie '0i/'er1 .!3..e1ee.ir1,elrrdes...e-.remeber @Oelregseeten f vvhich may be provided with fans.orfoolin'g coils, er..beth...rererfeeting eee1ir1e..ef..rhe sheet ingeert .inierve1eitime- Thesrevisien er e @e01- .irre.z...eer1e....immedietely... renewing ...the ...heeiei .eerirrerenes .else eisisin...s eeurirreeeeurere eers..- trel .ef the .eurir1g-91eeration- ...Itwill .be underv .SOQQL that.. .other means .and procedures.. can, 4be.employed .for .ineerperating .ther resimime .the sheetnerrdreffeetirieihe. curing Qf. the .seme inthe.epparatusillustrated inFigurel; aelue line adhesive is appliedto one f acef the cured.resin-.bearing sheet; .immeeietelyerterit leaves .thepvenherrleerrsfereeeemplishirg'this .mehr esererrr! reneerrteinine the. adhesive irrliquid form, a dip roll 22 and a pair of squeeze rolls .23 and 24. Afterpassing through the adf- ,i The glue line adhesive can be applied as acompletely separate operation, or it may be )omitted entirely. In someinstances, it will be .found more desirable to apply the adhesive forjoining the masking sheet to the surface of the .plywood or otherproduct with which the sheet is used at the time that the sheet is to beattached and the adhesive may then be applied to .the Wood rather thanto the masking sheet, or a separate sheet or film of the adhesive may belused in accordance with the known practices of the art.

The base sheet used in apparatus of the type shown in Figure 1preferablyy comprises a 100% .kraft sheet.` However, satisfactory sheetscan be manufactured from pulp furnishes of other types, for example,sheets manufactured from sulphite pulp, ,semi-chemical pulp,groundwoodsulphite mixtures, etc., such sheets giving good results inmost instances. The `particular pulp or pulp mixture used will depend tosome extent vupon the handleability conditions required in the `final`sheet and upon the degree of scuff and .abrasion resistance required inthe end product.

Investigations have shown that the base sheet should have a basis weightwhen dry within the range of from about lt-o 210 pounds per standardream, 500 sheets 24 by 36 inches (bone dry basis), with a density Withinthe range of from about .55 .to .80 gram per cubic centimeter and thatthe optimum sheet has a dry basis weight within the range of from about125 to 135 pounds per standard ream, at densities Within the range offrom .65 to .'75 gram per cubic centimeter. If

the density is too low, the resulting masking sheet will lack abrasionor scuff resistance, and if the density is too high, it will becomeunduly Vdiillcult to accomplish the particular kind of resinimpregnation that is required in the attain- .ing of the objects of theinvention. Generally stated, the base sheet should be a relatively`dense sheet which, at the same time, is absorbent. 'Ihe heavier basisweight sheets tend to give lbetter masking, but sheets having basisweights ,materially greater than the optimum range specified areuneconomical.

. The caliper or thickness of the base sheet may vary between ratherwide limits, but the caliper lis desirably in the range of from about 10to 13.5

thousandths of an inch. The pH of the base sheet is not particularlyimportant, although it will usually be found most convenient to operatewith a base sheet which has a pH which is some- ,:whatnon the acid side,i. e. a pH in the range of from about 4.0 to 4.5. The tensile strengthof the `sheet is also not of too great importance, but it should besufficiently great so that the sheet may vbe drawn through theimpregnating apparatus IWithout tearing or breaking.

In one particularly satisfactory embodiment :of

Ythe invention the base sheet constituted a 100 per vcent kraft sheet,having an unimpregnated basis 'weight of approximately 125 pounds perstandard ream of500sheets 24 by 36v inches (bone dry basis). The sheethad an average caliper or thickness Within the range of about 12.5 to13.5

one-thousandths of an inch and a density within f the range `of from .56to .65 gram per cubic centimeter. The raverage net wet tensile strength.of the sheet was about 10 pounds per inch of width. The resinimpregnating material used in the manufacture of this embodiment of theinvention consisted of an aqueous solution of a phenol formaldehyderesin having high penetrability and low viscosity at the concentrationsused for impregnating the sheet, and the actual impregnating solutionhad a solids content of approximately 25 per cent and a viscosity of`about 50 centipoises at the impregnating temperature, which was withinthe range of from about to F. Elevated temperatures of this order aredesirable during the impregnating operation in order to improve .thepenetration of the resin into the base sheet and to facilitate thesubsequent drying operati'on. Y. l

After passage through the squeeze rolls III and I2, the impregnatedsheet had a wet weight of approximately 365 pounds per standarddream.This resulted in a resin content in the nal dry sheet, as it came fromthe oven I3, of approximately 55 pounds per standard ream, or a resincontent of about 30.5 per cent, based on the finished total weight yofthe dry sheet. i In the oven I3, the initial zone I'I was maintained ata temperature of about 325 F. Vflf'he temperature of the next succeedingzone I8 was approximately 350 F. and the temperature in the nal heatingzone I9 was maintained at approximately 375 F. The entire oven I3,excluding the cooling zone 20, had a length of approximately 100 feet,andthe three heating zones were of equal length. The temperature in thecooling zone was maintained substantially equal to room temperature bythe use of circulating fans. During the entire operation, the sheet wasmoved at a uniform speed within the range -of from about 25 to 30 feetper minute. Under these conditions, the resin was dried and cured withinthe oven structure I3 to the highly advanced state which is required incarrying out the invention. A portion of a completed sheet isillustrated at 29 in Figure 3. It will be understood that theimpregnating resin is distributed throughout the base sheet and that theglue line resin comprises a thin surface coating. f Y

In another highly satisfactory embodiment of the invention urea-aldehyderesins were employed. Urea-aldehyde resins are either white orsubstantially colorless when cured,l and when these resins are used,masking sheets which are light in color may be produced. Such maskingsheets contrast with the relatively dark sheets which result from theuse of Phenol-aldehyde resin in the masking sheets. In order to obtainthe maximum advantage from the use of ureaaldehyde resins, base sheetswhich are light in color should be employed, as for example, sheetswhich are made from unbleached sulphite pulp, bleached sulphite pulp,semi-chemical pulp, groundwood pulp, alpha sulphite pulp, soda pulp, andbleached kraft pulp, or mixtures thereof. course, sheets made from otherpulps may be employed if the color of the sheets is not objectionable.

A masking sheet containing a urea-formaldehyde resin was produced bytaking a selected base sheet made from all sulphite furnish (unbleachedbalsam sulphite) having a basis weight, when dried, of approximately 119pounds per standard ream of 500 sheets, 24 by 36 inches (bone drybasis). The sheet possessed sufficient wet strength to enable it to bedrawnthrough the impregnating bath and thedrying oven, and

, zer'rresinv content, However, such-flisht f st '"f lmarmfafctureandflandlealiilty of the 'sheet "ae fnottoo'fimportant; the resincement' een be rirv`creased to'fas 'highas-B per'fcent. The us'pfsheetsfhalving'a resin content `onacry sheet Al'niajsis of 1lessVthanvl'O tper cent or greater than'ffpr cntfis fnotfgenerallyrecommended.

Thefamo'unt `of `'uncured' resin 'applied" t'o-the base ;sheet 'duringthe impregnating operation should p e suh, -When--nreaformaldehyderesins are employed, to produce laconcentrationV 'of eure@ resin in theimpregnated andl ldried sheet @11.1 the. range. of imm. 5 it@ 535 .--percent A,by .Wght preferred resin content 'the caseAl or phenolic Iresinimpregnation, is-within trie range of-froin about 2'5 to SOfper cent byWigi'ihro'n abne dry basis.

The d'ensityof the i'she'diinasking sheet as nico es jfrein "or tbfs"bwww used, is'a particularly ugem index brine exenicef the sheet.Generally stated, the maskingsheet at this time, should have a densitywithin the range of from about .60 to .95 gram per cubic centimeter, andsheets within this range Will have satisfactory Wet and dry abrasionresistance. In

`this connection, it is important to note that this density range isattained Without the application of pressure during the curingoperation.

It will be noted that the carrying out of the impregnation processes onapparatus of the type shown in Figure 1 causes the resin reaction to becompleted and the resin to be formed within the cellulosic web of theVbase sheet." This is believed to be of some importance in accomplishingthe objects of the invention, and especially increasing wet abrasion, bythe procedures hereinafter set forth.

The `drawings also illustrate apparatus for preparing a masking sheet bythe addition of all or part ofthe resin-impregnation to the pulp priorto formation of the sheet, this apparatus `being illustrated in Figure2.' 'I'he incorporation of all or part of the resin into the pulp hasseveral distinct advantages. In the first place, such ja ymethod isinherently economical because the resin is incorporated into the sheetWithout the necessity of an additional step and the need forimpregnating apparatus is eliminated. In the second place, the resin canbe uniformly distributed in the pulp through the inherent operation ofthe beaters, which are normally employed in preparing the pulp for thepapermaking operation, independently of the characteristics of theresulting sheet.

In the particular apparatus shown in Figure 2, a beater, which isemployed in the usual manner, is illustrated generally at 3|. Duringopthroughout the paper furnish. After the pulp and resin are thoroughlymixed,

the contents of the beater are delivered to a pa- 'permaking machinewhich maybe the Fourdrinier 'type as shown at 33. The furnish istransferred from the beater 3| to `the papermaking machine lvia theusual stock chest 35, and stock pump 31,v which, may be ofthe-centrifugal type, as -ilillustrated. From the stock pump 31, thepulpV passes through a Vdistributing iiow box 39, ma-

chine screens 4|', and flow box 43 in accordance with known practices.The masking sheet formed onl the Fourdrinier wire 44 is conveyed through4thefusual press rolls 45 into `a drier section 41.

Afterpassing through the dryer section, -the sheet may be passed throughcalenders 49, and

' finally it is wound upon a take-up reel 5|.

lIf a glue line is to be applied to one face of the resin-bearing sheet,the line maybe applied after the sheet leaves the calenders 49. l Asuitable `means` for accomplishing the rapplication of Atheglue line isshown in the illustrated apparatus, and includes a pan 52 for containingthe adhesive in liquid form, a dip roll 53 and a pair i of squeeze rolls55 and 51. After passing through the adhesive'applying meansthe sheet isconducted through a drying chamber 59,which is operative to dry theadhesivev on the sheet merely to a non-tacky condition. .Afterapplication Y of resin the glue line, the sheet may be taken up on thereel 5|.

.As in the case' of the impregnated sheet resulting from the immersionprocess, the glue line adhesive can be applied as a completely separateoperation, or it may be omitted entirely. When it is omitted, theadhesive for joining the masking sheet to the surface of the plywood, orother product with which the sheet is used, may be applied to thesurface of the wood or a separate sheet or film of adhesive may be `usedintermediate the masking sheet and wood in accordance with Well knownpractices. Generally, however, better masking characteristics areobtained when the glue line adhesive is applied to the sheet and driedto a non-tacky condition.

In the manufacture of resin-containing maskingsheets in accordance withthe invention, by processes'whereby all or part of the resinimpregnation is by beater addition procedures, the sheet may be made bythe use of almost .any of the known types of pulp. For example, I havefound that unbleached kraft pulps, unbleached sulphite pulps, groundwoodsulphite pulp mixtures, and semi-chemical pulps provide highlysatisfactory results. While the results obtained are not dependent uponthe kind of pulp, they are directly related to the pulp freeness. Forobtaining most satisfactory abrasion resistance in the resulting maskingsheet, lower pulpV freenesses are desirable. However, if the pulpfreeness is too low, the handleability of the resulting sheet may beimpaired and the sheet will be more diilicult to manufacture. Mostsatisfactory masking sheets are obtained when thefreeness of the pulplies in the range of from about 300 to 400 when measured by the Canadianfreeness tester. 1

I vhave found that particularly good results can be obtained throughtheuse of a pulp comprising a mixture of chemical pulp such asunbleached balsam sulphite and groundwood. In general, pulps of thedesired freeness are obtained when the ratio of chemical pulp to ground-Wood is in' therange of from about 50 to 75 parts of chemical pulp tofrom'about 50 to 25 parts groundwood. f

When comminuted insoluble resins are employed as beater addition resins,such as a water insoluble phenolic resin, further treatment of the pulp,after resin addition is not required. However, when water soluble,alkaline soluble resins, or resin emulsions are used, it will generallybe necessary to acidify or otherwise treat4 the pulp 'in order todeposit the resin upon the fibers comprising the pulp. If the pulp isnot acidied, the =resin may not bind satisfactorily to the pulp and,consequently, will drain from the pulp in the paper making operations'.`Accordingly, after a soluble resin or resin emulsion is added to thebeater, the Vusual Apractice will be to reduce the pH of the pulp toavalue from between aboutV 4 and about 7 through the addition of anacidifying agent such as alum,-sulfuric acid, mixtures of alum andsulphuric acidI mixtures of acetic and phosphoric acid, etc.

The addition of the acidifying agent causes vthe resin to comev out `ofsolution and deposit upon the cellulose fibers, In the case of i somebeater addition resins, this deposit constitutes a precipitation but, onthe other hand, when other beater addition resins are employed,acidification results in the breaking of an emulsion or colloidalsuspension causing the resin to coagulate upon the vbers.l

Generally speaking, thelower the pH, the greater the percent of resinretention .by the iib'ers. However, lower pH values interferewith thepulp processingand paperrnaking equipment. llorrbesti results, thepulpshouldnbeacidied to a pH iny the range of:4;5 to 5;5.

Best" masking `qualitiesl result when the' basis Weight ofv the sheetmade! by the use" of `a vresin containing'pulp is relatively high,:inthe range `oli'rom. about 150i to =300fpounds per 500.- ream ofsheets-.241W 36 inches (bone dry basis). The thiokness'ofthe'vsheetshould' be in thefrangeof -fromfabout 141 to .28. mils.However; in' some instances, :proper equipment. may notl be availablefor-obtaining -sheetsY of suoli high basis weight necessitatingl themanufacture of sheets 'having much. lower :basis weights. 'If suchlightweight sheets: are'. to bei satisfactorily used for' maskingpurposes, multiple llayups willl ordinarily be used, ine..several?ofthe`sheets will be superposed upon one another, andi the individual sheetsmaybe 'heid togetherby' means of a lightglue lineal;-

to one side of. the'sheet or by means"v ot a separate-'attachment sheet.

The resin content of` themasking sheet'im- Ypregnatedby the beateradditionV processmay "vary between rather broad limits, depending uponthe properties 'desired in the' resulting sheet. Generally, i thelresinl content of the sheet is. directly*proportionalito the. amountof"ahrasion resistentie, kwai-,erA resistance, etc. However, if thev`resin content of the sheet-is too high, hanelleability of the sheet islowered andthe mask- `ing?sheet'fibecornes'tooexpensive. It has been'found thatoptimum overall results are obtained 'when thermal sheet'contains between about 25 'percent amd'aboutao percentgresin, onxa bonedry basis, as in the case of resin impregnated `fslreets prepared bytheimmersion process. However, a' masking sheet satisfactoryV formanypurisattainable 'with resin contents as low vas Aabout 1'0 per cent. Theresin content should not bei overabout 35 per cent.

llt will. be understood, of course,` that.substanftiallyfgreaterfamounts off resinv -rwill'have to be fadded 'to''the beater than is contained' 'in the resulting sheet to compensate forinherentresin solubility, 'physical 'lossesy etc. Investigations haveshown that the 'losses may equal the amount fof resin 'in the maskingisheetfand, accordingly, twice as 'ni-uch res-in should be added tothebeater islpreserrt iin the vfinished, sheet.

The additionk resinsemployed'zpreerably lconfstitutel one yor 'morefof-fthe relatively rapid curing Such characteristics are. importantbeca-use the resins should' advance Sto highdegree oicuref during:manufacture Lof the sheet. 'In "ftl'is'fconnection, iteshould be notedthat hereto- *ffore it has -b'een considered 4undesirable to permit`tl'ie"'res"in sheet material for laminating f-purposes, fto advance incure and such a result was avoided. natersoluble resins, or .resinssoluble in- 'alkaline solutions Jare '-preerred, `'and .there :arevarious gsuch phenol 'formald'ehyde resins avail- --afble`4cornrnereiiatlly. 'Water dispersableresins lin emulsion or drypowdered form mayalso be used.

*It-will be understood `thaturea formaldehyde and melamine resins'having satisfactory fc'hartaeter'isticsfm'ay 'also :be used. In-the'latt'er coninecti'on, investigations 'have shown that fitis verylillciilt 't'o obtain sheets containing more ithan ellooi't 56 -res`in4f'tbon'e 'dry ibasis) when Ypresently availablebeater addition .resinsof fthe urea and fmlaininetype are employed. '.nccordinglyfitfmay benecessary to incorporate one or more-'addil2 tional.- resinsy hrsuchsheets Lbrother impregnationprocedures,` as for example Vbytheprocedures hereinbeforei-described, toobtain a sumcient total resin;content to provide a` satisfactory masking sheet.

:When resins are' added totheV beater in Comminutedf. solids form. theytend. to form 4gliibulfs in theresulting masking sheet and, theseglobules are apparent in thermaskng sheet, thereby decreasing therna-rketabilityI ofthe sheet.. Therefore, the-resin. is preferablyadded'vto thenulp in liquidv'form and for this reason. water or alkalinesoluble.- or emulsion forming resins are employed.

'In. the manufactureV of a 'satisfactory embodiment of .thenin-venton bythe beater addition resin-impregnation procedure, a beater mixtureconsisting of 1170; pounds' of unbleached V11emlockfsulphite pulpand 630poundsof unbleched sprucergroundwood was prepared. To this was .addedanalkaline solution of.- a kpher-iol-aldehyde :resin` lof .the beateraddition type.: the solntimri comprisingabout Y- 46% resin solids..Sufeen-t solntionwas added-to provide900 poundszo resin. The resinsolutionv was allowed to mix with, the pulp in the beater forapproximatelyll) minutes and anaqueous solution-.containing .25 per centalum was-.added slowly tothe mixtureof pulp and resin'until thepI-I .ofthe beater @entente was iieduoed. to between 4.5 to 5.0.. Thestok had .afreenessofzeto 352` (C-anadiani-reeness tester) afterlbeating, and Wasidelivered directly. to `a papennaking machine. Dependingnpon variousoperating speeds and machineondions, the

resulting masking sheet had a basis` weight between 104- and 148 poundsper standard ream and a resin content of between 13 and ..277 yper cent..The -densityof the sheet varied between 6.3.v and .72y igram percubiccentimeter .and thefthickness .or vcaliper. `ofthe sheet varied between19,3 and 27.3 one-.thousandthsf-of an As before noted, the resin is`cured to a highly advanced state prior .to taking up. of. .the sheet onthe ,reel I The masking sheet prepared by the.. use .of addition resins.thus includes .a resin impregnation havingfa. .state .oiurcSubstantially .thesameasthose .resulting when the :Resin impregnation isincorporated into .the Sheet by .other processes. ,.Moreover, as .beforenoted,

.sheets .made by any of/-theprocessesare orten times ,d'icult .todilerentiate.

As has 'been indicated` above, thefmasking .sheet yof .the .invention,may .be .preparedby .a lcombination of thefaboveprooesses. Such acombination procedureis desirable whenit is advantageous to.incorporatetwo types .o f resins in `the masking sheet,as...flor..example.z-a phenol-.aldehyde resin and .a i-urea-laldehyderesin. may benone by .add-ing .a .beater .addition resinto-the pulp and,theniurther impregnating thesheet made `from the v,nullo with`anotl'ier Lyne 0f. resin by .an immersion .or other process. When thisp rp- .cedure isemployed, the'freeness. of the pulp .should lbesomewhaltlhigher than before indicated to assure adeguate and uniformimpregna- .tion .of the second resin. yHowever, the -freeness should not'beyso great as to interfere with the abrasion and -scu'ff resistingproperties 'of vthe sheet. *It lappears that Athe proper freeness 'range`for the usual Ymultiple type impregnation should be'fromabou't 400 to500, when measured `by the 'Canadian 'Sfreeness tester. l'It =wl1.=be:apparent lthat'two for v:more types of; resins 'may also beincorporated int-o ,fa .ibase zsheet 'by carrying :out theai'rmmersionimpregnation procedure `Suc- ,abrasion resistance.

cessive steps. It is desirable to advance vthe cure of the resin to ahigh degree before carrying out a second impregnation process but, undersome circumstances, two or more types of resins may be applied to a basesheet in seriatim prior to curing.

. As fully described in my aforementioned copending application SerialNo. 116,810, where urea resins are employed in the treatment of themasking sheet, very remarkable improvement in .the wet-abrasionresistance of the masking sheet material can be obtained by theintroduction vinto the sheet, preferably before the impregnation of thatsheet with resin, of small percentages 4of specific water soluble,inorganic acid salts of aluminum and ammonium. Specifically, thesecompounds are ammonium chloride, ammonium phosphate, ammonium sulfate,ammonium thiodesired increase in wet-abrasion resistance is to 1 beobtained. It isalso important that at least a part of the catalyticmaterial, and preferably all of it beadded to the base sheet prior tothe resin-impregnating operation, this being particularly true whenusing aluminum sulfate f which is the best of the 4several catalyticmaterials described. W'hen a catalytic material oi the type describedabove is used, the total amount -ofsuch material present in the sheetduring the resin-curing operation should be within the range of from .4to 1.5 per cent of the dry weight of the resin in the base sheet. Theoptimum amount of aluminum sulfate is within the range of` from .4 to .8per cent. A lesser amount of the catalytic material, or a greateramount,will .substantially decrease the resulting benecial effect and willresult in a sheet of much less Part of the catalytic material may beintroduced into the base sheet as a component of the impregnating resin,although the resultant sheets areusually somewhat inferior inwet-.abrasion `resistance to the sheets obtained when all of thecatalytic material is added to the base sheet prior to impregnation.Also it is possible to add all of the catalytic material to theimpregnating resin, although this practice results in a much lessywet-abrasion resistance sheet than' is the case when at least part ofthe catalytic material is added to the base sheet. Y

. The catalytic materials disclosed are all water soluble materialsl andmay be conveniently introduced into the basesheet by passing the sheetthrough a water solution of the catalytic material. Generally, whenAusing base sheets of the 'type described, the concentration of thecatalytic salt in the water solution should be about half `theconcentration required in the base sheet.

Aftery passing the sheet through the solution of vcatalytic material,the sheet is desirably, but

not necessarily, dried prior to the impregnation with resin.

The degree'of curing which is accomplished when the resin-bearing sheetmoves through the heated zones of the oven I3, or when themasking sheetis formed from resin containing pulp, is of the greatest importance inaccomplishing the objects rof the invention. In Aorder that thecompleted sheetshall be capable of masking the y v `various defectsdiscussed above, the resin conbe impractical or impossible to use.

14 tained in the sheet must be cured to such adegree that it willexhibit essentially non-flow characteristics when subjected to thepressures and temperatures normally employed in the manufacf ture ofplywood, i. e. to 300 pounds .per square inch and from about 260 to 325F. The cure must also be carried up to the point where the sheet isnon-laminating at pressure in excess or 1000 pounds per square inch at atemperature in excess of 250 F. Unless curing is advanced to thisextent, raised grain, patch outlines and splits will show through thecover or masking sheet, and the principal object of the invention, whichis the production of a smooth, blemish freesurf face, will not beaccomplished. However, the cure is desirably not carried to the pointwhere the sheet would be non-laminating at pressures over 2000 poundsper square-inch and temperatures over 350 F. If the cure is carried verymuch beyond this limit condition, the nal sheet may be too brittle tohandle in roll form and may As used herein, the term` non-laminatingrefers to the inability of the resin containing sheets to vadhere to oneanother. Thus, if the resin containing sheets are placed in face to facerelation and subjected to pressure and heat, as specied above., and donot adhere, the sheet is said to be nonlaminating under thoseconditions.

The desired procedure of the invention requires that the cure be carriedto the point where4 masking will be obtained, but not materially beyondthat point. The determination of when the` cure has proceeded to thisoptimum value, in the case of urea-aldehyde and melamine resins, can bereadily accomplished by a mechanical testing procedure which will bedescribed hereinafter in complete detail. When the resins employed areof the phenolaldehyde type, the state of `cure may be ascertained byeither this mechanical test or by a chemical test which will also be setlforth hereinafter.

Cover or masking sheets manufactured as described above, or byequivalent processes, can be conveniently applied to the .surfaces ofplywood sheets in a conventional plywood hot press. Such a press isillustrated schematically in Figure 4, andas shown in that iigure,includes a plurality of relatively movable, flatsurfaced platens 55disposed between top and bottom members 51 and 59, which also have flatplaten surfaces thereon. Means are provided Afor heating the variousplaten surfaces, and when the press is in use,

4the plywood or other material'is pressed between sure. As previouslynoted, pressures of the order of from 150 to 30G pounds per square inchwith platen temperatures of the order of 260'to 325 E are ordinarilyemployed in plywood hot presses; The masking sheet can be applied to asingle veneer, or it can be applied to a complete plywood assembly lofmultiple cross grain veneer, and in either case, the masking sheet maybe applied to either one or both surfaces. In the case of a plywoodassembly, the cover or masking sheet is most conveniently joined totheother elements of the assembly during the manufacture of the plywooditself, but it can be attached to a completed sheet of plywood as aseparate operation.

' Regardless of the particular combination which is being effected, itis`desirable to introducethe masking sheet into the press in such mannerthat it will be in contact with a caul'plate fi'i's evaporated undercontrolled conditions, and

`the residue remaining constitutes the extractable portion. Then bysimple mathematics, the percentage of extractable resin can becalculated.

Similarly, extended tests and investigations have shown thatsatisfactory masking will result l provided that the maximum quantity ofphenoliclike resin material which is extractable from the cured maskingsheet, as described, does not exceed a percentage figure which is equalto four plus i r the change in cure, expressed as a whole number sheetand not on the resin content alone.) Stated inanother way, a resin sheetimpregnated with a phenolic resin in the manner described will bevsatisfactory for masking purposes if, upon additional curing of thesheet for one minute at the temperatures and pressures employed in themanufacture of the commercial plywood products, it contains four or lesspercent of extractable resin.

-In determining the percentage change values,

it is most convenient to plot a curve of the percent extractable resinagainst the time of cure, and illustrative curves of this typeare-illustrated in Figure 8. Data for curves of this type is obtained byselecting a series of samples ofthe base sheet containing the resinwhich is to be evaluated, and curing the samples to various degrees byplacing the sample sheets in an oven for various intervals of time, theoven being maintained at the temperature vat which the sheet isnormallycured. Following the curing operation, the per cent of extractable resincontained in each of the samples is determined according to the ASTMprocedure described above. A plot or graph may then be obtained,illustrating the variation in the per cent of extractable resin against`curing time as illustrated in Figure 8.

To determine the minimum permissible degree of cure `from one of thesecurves, a line is drawn i parallel to the abscissa at the ordinate valueequal to-4 per cent extractable resin. Then, by extrapolation as shownby the dotted lines in Figure 8, the amount of cure obtainable in theminute preceding the arrival `at 4 per cent extractable resin can beaccurately determined. This value, addedto the empirically determinedconstant 4, as described above, will give the maximum percentage ofextractable resin which can be tolerated in the masking sheet. It willbe found that theV permissible maximum amount of extractable resin willvary, depending upon the particular resin or resin mixtures used forimpregnation. For example, a fast curing resin or resin mixture,` asillustrated in the curve numbered I in Figure 8, will give good maskingwith a maximum extractable resin percentage of 12.4 per cent, whereas aslow' curing resin or resin mixture, as illustrated in the curvenumbered III, requires that the cure be continued to a point where themaximum extractable resin is no greater than 4.9 per cent. If thesamples are cured at the same temperature as that used in the curingoven for the masking sheet, the proper curing time can be determineddirectly therefrom. Otherwise tests,v may be required.

When the masking sheet is made from resin containing pulp, it has beenfound more desirable to employ a mixture of 5 parts hydrochloric acidand parts acetone for extraction purposes in the above chemical test.However, if such va mixture is used for extraction purposes then thestandard A. S. T. M. extraction procedure described above ean not beused because of the difference in vapor pressures of the components ofthe solution. The extraction procedure used in such instances then is asfollows. A weighed sample of the material is suspended in a hot mixtureconsisting of 95 parts acetone and 5 parts concentrated hydrochloricacid and occasionally stirred. After about l0 minutes the sample isremoved from the acetone hydrochloric acid mixture and suspended in ahot, fresh solution of acetone hydrochloric acid comprising the sameproportions as in the first extraction step. After a second ten minuteinterval with occasional stirring, the extraction step is repeated athird time. The sample is then removed from the mixture, air dried toremove excess acetone, and dried in C. oven for 10` minutes. The weightof the dried sample is then determined. From this Weight together withthe initial weight of the sample the per cent of acetone extractablematerial can readily be determined by simple calculation.

In order to obtain increased wet strength in the base sheet used in themanufacture of masking sheets in accordance with the invention, it willsometimes be found desirable to add to that sheet a wet-strength resin,or other material, in accordance with known practices in the papermakingart. Some of these resins are of the beater addition type, althoughothers are'added to the sheet by impregnating operations. Insofar as theproduction of the masking sheets of the present invention are concerned,the presence or absence of these wet-strength producing additives in thepercentages normally employed (1/2 to 3% of the dry Weight of thecellulose),

has little eifect. Also, it makes no difference Whether the additive isof a resinous or other nature. Similarly, the base sheet may containpigments or printing, if decorative surfacings are to be produced.

The cover or masking sheets described in the foregoing make possible theattainment of a very smooth, easily finished surface on plywoodjandsimilar materials. The surface vprovided may be either light or dark incolor; Vit may include decorative effects; and it will receive paint andenamel finishes particularly Well. Of even greater importance, from acommercial standpoint, the masking sheets of the invention 4effectivelymask plug patches, grain raising and checking, and can be manufacturedand applied to` the plywood or other sheet materials withv which-theyare used at relatively low cost.k

Various of the features ofthe invention are set forth in the followingclaims.

v I claim: Y p

1. The improvement in the process of manufacturing brous sheet coveredplywood the face of which plywood adjacent the fibrous sheet containssurface defects, which comprises the steps of impregnatingi a papersheet having a basis weight, when dry, within the range of from to210pounds for 500 sheets 24 by 36 inches, and

a density within the range of from .55 to .80 gram per cubic centimeter,with a thermosetting Yaldehyde resin to provide a resin content in saidsheet, atfer drying, within the range of from about 5 to- 35% of theweight of the dry impregtaining sheetand i9 thatyvhena action of y the.,cnredsheetisgpressed jtiyeriaf essentie-basking-plate-andaiiat, Letali'i'c. die-biete. @bel-1 1 .approximately- .-inones (inches sanare,said @die plate .having o-ver eIg-reafter ,portion .of its .surface,concentric 4.anfiiiehiiiiNvidia spaced apa-ruby coneertric Vgrooves .fof :aninch in `.v fiith, with fafoce of Vabout 12,300 pounds, uniformlyI:disbtedlbveilthe plate, lthe sheet being ,heated r gLthe pressingoperation ,to a itemperature jofrabout 300 WhileL-Inaintained.underpressure L f iba'jerio'd gof fsixfto pig-htgninutes, said sheet ree of'visiblelnrarls corresponding -,to .the die hat r1 hesidgthereof-ODDOSite to ithe side iii 4101i @et :Wihilie di? Serf-ace5r1acins at least .y -Of S9491 @Bred Sheet over the exposed .-face ofim? Qlkir 1V I 'lis O a Plywood assem- "Qli ieee semaine serio-oedefects, when essably is elated in a plywood press;- pro- -/ieine' .a e

n-eeef-r lly subi ure g Sheet and the adjacent veneer; 'ses'tesserati-ain said press,

t. anee O f .from about 150 i 3.99 rieur- 151s Seilere and a temperatureWithin the range of from about 260 to 3259 is? .dieci the ifi-ning ofthe plies of the oi-ywood e Y ali-,fi the attachment of the cured resin-QOP Iig sheet i9 the suriaee Veneer solely by the ve action of said eineline. 2V he Jment the process of manuf p fatigues tier-ees sheet coveredplywood the face -o-fwhicl PIYWQQQ. agissent the brous sheet contains""surface defeetsz which comprises the steps g a raser sheet havingabasis dry-, Within the range of .from about ,fer- 500. sheets `2 4 by 36inches, hin.- the.- renee, oi from .55 to. .80 'Mieter-, withthermosettine d resi- 1.1 @0 QiQ-Vide a resincontent in said 4, after,dryirig, Within the, range 0i`v from i? 5.` Q- 3@ Gf; the Weight Gfx'the dry impregd. Sheet-: drying the resineeentaining sheet. imireanaied:.Sheetha 'ne basis. weight dry Within the range ot-.frorn about :11u

vvhen @5..- grem- Per- Gabin centimeter-: heating said. dry,

lie mf-@Qntaiengf s .lieet to, cure. the resin conta `edthereintotsuchanv exten that when. a .101,105 fhfcllredisheetis pressed-'between a1.f1 metallie backing.) plats and.- a fia-t, meta-ino die plate,

cppjtagt i die-LSurface-si: piaeing. ati least:

allied-1sheetiover the exposed iface- 1Miersfoi1afplyyvoodfassembly Isurface; defects.- when that'- .-,afplywoodpresseprovidingedadhesivefintermedi-atestile-resin-cone the adjacent -veneer;-and -naily' subjectingisaid .assembly,-. in-said-fpress,topres-isurawithinthe range sof -fromaabout --150-to-300-' poundspeisquare ninch and a temperature Withinthegrange of--from about 26o-'to325 Fito eiect the joiningor -theplies of the plywood. assembly @dilleait'chment-b1ihee1ired resin.- contamme-- asin .e adhesive intermediatethe 9 both approximately 8- inches: by.- 81 L Lsheetto-.thesurfaceveeersolely by the adhesive @action ofsaid glue= line.

.-3. ,The ,improvement :in `the .--process of -manufaeturing -broussheetcovered plywood `the --fa-ce .of-Which plywood adjacent-the brous-sheetcontains f surface l defects `which comprises .-placing at Kleast oneply sof -a `masking -sheet -having -high ,Wet-andabrasion-resistanceover the-eXposed--face Lolf-,one of the outer veneers-of a plywood-assembly ,which face -contains surface defects when that:assembly is placed in a plywood ipress, said fmasking :sheeticonsisting of -a -cellulose fiber base sheet having -an -unimpreg-natedbasis weight when-drywithin the range -of -from about 110 to 21d-pounds:for 500 sheets :24 by 36 inches andan uni-mpregnated densi-ty withinthe range of from about .55 to l gram per Acubic -centi-meter, andcontaining from about 5 4to 35%-, based on the finished total Weigh-t ofthe dry, resinecontaining sheet, fof a thermosetting aldehyde resin,"said resin being :cured to such an extent that when a section of 'acured sheet is pressed between a at, metalli-c, backing plate and a dat,metalli-Tc, die plate, both approximately 8 inches by 8 inches square,said die plate having over the greater portion of itsl surfaceconcentric lands 5/{34 of an inch in width, spaced apart by concentricgrooves si; of an inch in width, with a force of about 12,800 pounds,uniformly distributed over the plate; the sheet being" heated during thepressing operation to a temperature of about 300 F. and being maintainedunder pressure for a period of six'to eight minutes, said sheet is freeVf visible marks corresponding to the die patternon th side thereofopposite tol ther side in cntaict with the die surface, providing a g-lelin'eadhesive intermediate thecured resin-*containing masking sheet andthe adjacent- Veneer; and nally sub-f jecting said assembly, in. saidpress, to` pressure- Within the range'of fr'ornf about 150' to300 poundsper'V squareinch-x and' a' temperature withirl the" range of from' about260 to 325" F. to' eifect the` joining of the plies of the plywoodassemblyr and the attachment of. the'v curedr resin-'containing masking;sheet: tot the surface' veneensblely' by thefadhesive action'-ofrsaidfglueline.

4., The improvement.- iny the" process off manur facturing fibroussheetl covered: plywood the face of: which plywoodv adjacent` the-lbi'ous sheet? contains surface: defectsk which' comprises pla'cl" ingati leastone:k plyY ofV a' masking;- sheet'- Having' high Wet" andiabrasion resistaiieeover the ex"- posed i face of. one' ofi thefoterfveneersof a" ply" press;` said rmaskingesh'eetf-consistingf of acellu'' losef ber(basersheetrwhiehfis-impreghated with arthermosetting.:aldehyde resin; said ibasel sheet-f having.r an.- unimpregnatedbasisl'weight; when dry,- within fthefrange of from@ about 110y to '210 poundsfor 500-sheets--24 by.- 36 inches,-l and anunimpregnated densityWithin-the range of from about .55 to .BOfgram-sperautrici-centimeter;said'-l impregnated sheetcontaining from@ about 5 to-Y basedv on-thef-nishedtotal-iweight of--theA dryimpregnated sheet,-.- oftheimpregnatingy resin, said.` impregnatedsheethaving a -basis Weightwhen'drywithin the-rangeof from about .60` .toA.95.,gramiperacubiocentimeter, and-said resinbeingocured.,to=such-anextent that When-- a section ofi-*the ,curedsheet --ispressed-between aflat, metallic, backing vplateand a flat, metallic,. die plate;bothiapproximately.,V S-inches byf8 -inches square, .said -die plate"having Vover the greater. portion -of its sur-face concentric 2T. lands5A, of an inch in width, spaced apart by concentric grooves g3g of aninch in width, with a force of about 12,800 pounds uniformly distributedover the plate, the sheet being heated during the pressing operation toa temperature of about 300 F. and being maintained under pressure for aperiod of six to eight minutes, said sheet is free of visible markscorresponding to the die pattern on the side thereof opposite to theside in contact with the die surface, providing a glue line adhesiveintermediate the cured resin-containing masking sheet and the adjacentveneer; and nally subjecting said assembly, in said press, to pressurewithin the range of from about 150 to 300 pounds per square inch and atemperature within the range of from about 260 to 325 F. to effect thejoining of the plies of the plywood assembly and the attach- 22 ment ofthe cured resin-containing masking sheet to the surface veneer solely bythe adhesive action of said glue line.

DONALD H. GRANGAARD.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date Re. 23,347 Redfern Mar. 20, 1951 1,857,690 Mellanoff May 10,1932 2,031,260 Gams Feb. 18, 1936 2,325,302 Britt July 27, 19432,343,740 Birmingham Mar. 7, 1944 2,383,283 Auxier Aug. 21, 19452,442,422 Loetscher June 1, 1948 2,497,712 Auchter Feb. 14, 19502,565,251 Malmstrom Aug. 21, 1951

1. THE IMPROVEMENT IN THE PROCESS OF MANUFACTURING FIBROUS SHEET COVEREDPLYWOOD THE FACE OF WHICH PLYWOOD ADJACENT THE FIBROUS SHEET CONTAINSSURFACE DEFECTS, WHICH COMPRISES THE STEPS OF IMPREGNATING A PAPER SHEETHAVING A BASIS WEIGHT, WHEN DRY, WITHIN THE RANGE OF FROM 110 TO 210POUNDS FOR 500 SHEETS 24 AND 36 INCHES, AND A DENSITY WITHIN THE RANGEOF FROM .55 TO .80 GRAM PER CUBIC CENTIMETER, WITH A THERMOSETTINGALDEHYDE RESIN TO PROVIDE A RESIN CONTENT IN SAID SHEET, AFTER DRYING,WITHIN THE RANGE OF FROM ABOUT 5 TO 35% OF THE WEIGHT OF THE DRYIMPREGNATED SHEET; DRYING THE RESIN-CONTAINING SHEET; HEATING SAID DRY,RESIN-CONTAINING SHEET TO CURE THE RESIN CONTAINED THEREIN TO SUCH ANEXTENT THAT WHEN A SECTION OF THE CURED SHEET IS PRESSED BETWEEN A FLAT,METALLIC, BACKING PLATE AND A FLAT, METALLIC, DIE PLATE, BOTHAPPROXIMATELY 8 INCHES BY 8 INCHES SQUARE, SAID DIE PLATE HAVING OVERTHE GREATER PORTION OF ITS SURFACE CONCENTRIC LANDS 5/64 OF AN INCH INWIDTH, SPACED APART BY CONCENTRIC GROOVES 3/32 OF AN INCH IN WIDTH, WITHA FORCE OF ABOUT 12,800 POUNDS, UNIFORMLY DISTRIBUTED OVER THE PLATE,THE SHEET BEING HEATED DURING THE PRESSING OPERATION TO A TEMPERATURE OFABOUT 300* F. WHILE MAINTAINED UNDER PRESSURE FOR A PERIOD OF SIX TOEIGHT MINUTES, SAID SHEET IS FREE OF VISIBLE MARKS CORRESPONDING TO THEDIE PATTERN ON THE SIDE THEREOF OPPOSITE TO THE SIDE IN CONTACT WITH THEDIE SURFACE; PLACING AT LEAST ONE PLY OF SAID CURED SHEET OVER THEEXPOSED FACE OF ONE OF THE OUTER VENEERS OF A PLYWOOD ASSEMBLY WHICHFACE CONTAINS SURFACE DEFECTS, WHEN THAT ASSEMBLY IS PLACED IN A PLYWOODPRESS; PROVIDING A GLUE LINE ADHESIVE INTERMEDIATE THE RESIN-CONTAININGSHEET AND THE ADJACENT VENEER; AND FINALLY SUBJECTING SAID ASSEMBLY, INSAID PRESS, TO PRESSURE WITHIN THE RANGE OF FROM ABOUT 150 TO 300 POUNDSPER SQUARE INCH AND A TEMPERATURE WITHIN THE RANGE OF FROM ABOUT 260 TO325* F. TO EFFECT THE JOINING OF THE PLIES OF THE PLYWOOD ASSEMBLY ANDTHE ATTACHMENT OF THE CURED RESINCONTAINING SHEET TO THE SURFACE VENEERSOLELY BY THE ADHESIVE ACTION OF SAID GLUE LINE.